One of the problems with scientific research is there's a sneaky bias that can creep in -- manifesting as explaining a phenomenon a certain way because the explanation lines up with a narrative that seems so intuitive it's not even questioned.
Back in 1978, evolutionary biologist Stephen Jay Gould nicknamed these "just-so stories," after the 1902 book by Rudyard Kipling containing fairy tales about how animals gained particular traits (the most famous of which is "How the Leopard Got His Spots"). Gould was mainly pointing his finger at the relatively new field of evolutionary psychology -- giving straightforward evolutionary explanations for complex human behaviors -- but his stinging criticism can be levied against a great many other fields, too.
The difficulty is, this bias slips its way in because these explanations seem so damned reasonable. It's not quite like confirmation bias -- where we accept thin corroborative evidence for ideas we already agreed with, and demand ridiculously high standards for counter-evidence that might falsify them. It's almost like confirmation bias, only backwards -- after hearing it, we experience a "wow, I never knew that!" sort of delight. We didn't already believe the explanation; but when we find out about it, we respond with open-armed acceptance.
One good example, that I had to contend with every single year while teaching high school biology, was the whole "right-brained versus left-brained personality" thing, which was roundly debunked a long time ago. It's certainly true that our brains are lateralized, and most of us have a physically dominant hemisphere; also, it's undeniable that some of us are more holistic and creative and others more reductionistic and analytical; and it's also true that the cognitive parts of the right and left brain seem to process information differently. Putting these three together seems natural. The truth is, however, that any connection between brain dominance and personality type is tenuous in the extreme.
But it seems like it should be true, doesn't it? That's the hallmark of a "just-so story."
The reason this topic comes up is a recent paper in the journal Global Ecology and Conservation that challenges one of the most appealing of the "just-so stories" -- that the reintroduction of wolves to Yellowstone National Park caused a "trophic cascade," positively affecting the landscape and boosting species richness and species diversity in the entire region.
The original claim came from research by William Ripple et al., and connected the extirpation of wolves with the corresponding higher survival rate of elk and deer. This, they said, resulted in overbrowsing of willow and alder, to the point that as older plants died they were not being replaced by saplings. This, in turn, led to higher erosion into streams, silting of the gravel bottoms required for salmon and trout to spawn, so a drop in fish population. Last in the chain, this resulted in less food for bears, so a reduction in survival rates for bear cubs, and a decrease in the numbers of grizzly and black bears.
The reintroduction of wolves -- well, supposedly it undid all that. Within a few years of the establishment of a stable wolf population, the willows and alders rebounded because of higher predation on elk and deer -- leading to a resurgence of trout and salmon and an increase in the bear population.
This all sounds pretty cool, and doesn't it line up with what we'd like to be true? The eco-minded amongst us just love wolves. There's a reason they're featured in every wildlife calendar ever printed.
It's why I almost hate to tell you about the new paper, by Daniel MacNulty, Michael Procko, and T. J. Clark-Wolf of Utah State University, and David Cooper of Colorado State University. Here's the upshot, in their own words:
Ripple et al.... argued that large carnivore recovery in Yellowstone National Park triggered one of the world’s strongest trophic cascades, citing a 1500% increase in willow crown volume derived from plant height data... [W]e show that their conclusion is invalid due to fundamental methodological flaws. These include use of a tautological volume model, violations of key modeling assumptions, comparisons across unmatched plots, and the misapplication of equilibrium-based metrics in a non-equilibrium system. Additionally, Ripple et al. rely on selectively framed photographic evidence and omit critical drivers such as human hunting in their causal attribution. These shortcomings explain the apparent conflict with Hobbs et al., who found evidence for a relatively weak trophic cascade based on the same height data and a long-term factorial field experiment. Our critique underscores the importance of analytical rigor and ecological context for understanding trophic cascade strength in complex ecosystems like Yellowstone.
MacNulty et al. demonstrate that if you re-analyze the same data and rigorously address these flaws, the trophic cascade effect largely vanishes. "Once these problems are accounted for, there is no evidence that predator recovery caused a large or system-wide increase in willow growth," said study co-author David Cooper. "The data instead support a more modest and spatially variable response influenced by hydrology, browsing, and local site conditions."
It's kind of a shame, isn't it? Definitely one of those "it'd be nice if it were true" things. It'll be interesting to see how Ripple et al. respond. I'm reminded of a video on astronomer David Kipping's wonderful YouTube channel The Cool Worlds Lab about his colleague Matthew Bailes -- who in 1990 announced what would have been the first hard evidence of an exoplanet, and then a few months later had to retract the announcement because he and his co-authors had realized there'd been an unrecognized bias in the data. Such admissions are, naturally, deeply embarrassing to make, but to Bailes's credit, he and his co-authors Andrew Lyne and Setnam Shemar owned up and retracted the paper, which was certainly the honest thing to do.
Here, though -- well, perhaps Ripple et al. will be able rebut this criticism, although having read both papers, it's hard for me to see how. We'll have to wait and see.
Note, too, that MacNulty et al. are not saying that there's anything wrong with reintroducing wolves to Yellowstone -- just that the response of a complex system to tweaking a variable is going to be, well, complex. And we shouldn't expect anything different, however much we like neat tales of How the Leopard Got His Spots.
So that's today's kind of disappointing news from the world of science. How we have to be careful about ideas that have an immediate intuitive appeal. Just keep in mind physicist Richard Feynman's wise words: "The first rule in science is that you must not fool yourself -- and you are the easiest person to fool."
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